A variable gain amplifier for controlling gain according to the change in level of an input signal is known in the art. As an example of the variable gain amplifier, patent reference 1 discloses a circuit in which a variable resistor means comprises a diode that is connected between the emitter terminals of a transistor constituting a differential amplifier that controls current flowing through this variable resistor means in such a way as to be proportional to the index of a control input.
Patent reference 2 discloses a circuit for controlling gain by changing the resistance value of a variable resistor that has as an input the two output currents of a differential amplifier circuit.
FIG. 4 shows an example of the variable gain amplifier circuit for controlling gain by changing current flowing through the MOS transistor of a differential amplifier circuit.
A variable gain amplifier circuit 10 comprises a current source 11, a p-channel MOS transistor 12 with a control voltage for controlling gain applied to the gate, a p-channel MOS transistor 13 with a control reference voltage applied to the gate, four p-channel MOS transistors 14-17 for differentially amplifying an input signal and its inverted signal, and resistors R1 and R2 which are connected to the drains of the p-channel MOS transistors 14 and 17 respectively and to ground.
The variable gain amplifier circuit 40 shown in FIG. 4 changes the gain of an amplifier circuit by controlling current flowing through the p-channel MOS transistor 12.
Next, the operation of the variable gain amplifier circuit 10 shown in FIG. 4 is described referring to FIG. 5(a)-5(c).
FIG. 5(b) shows the relationship between control current Idd1 and Idd2 flowing through the p-channel MOS transistors 12 and 13 of FIG. 4 respectively and a control voltage, in which the vertical axis indicates the control current level of Idd1 and Idd2, and the horizontal axis indicates the control voltage.
If the drain current (control current) Idd1 and Idd2 of the p-channel MOS transistors 12 and 13, respectively, become equal at a specific control voltage Va, as the control voltage decreases from this value, the drain current Idd1 and Idd2 of the p-channel MOS transistors 12 and 13 increases and decreases, respectively.
If an input signal level is low, a control voltage for increasing Idd1 is applied from a circuit, not shown in FIG. 4, to the gate of the p-channel MOS transistor 12. If Idd1 increases, the drain current Id1 of the p-channel MOS transistor 14 increases, and Idd2 supplied with current by the same current source 11 decreases resulting in a reduced the drain current Id4 of the p-channel MOS transistor 17.
Since currents flow proportional to drain current Id1, Id2 and the signal level through the resistors R1 and R2 respectively, the difference in output voltage between the resistors R1 and R2 increases to increase the gain of the variable gain amplifier circuit 10.
In FIG. 5(c), the vertical and horizontal axes indicate the gain of the variable gain amplifier circuit shown in FIG. 4 and a control voltage, respectively. The gain is controlled such that a decrease in the control voltage increases the gain if the input signal level lowers.
In FIG. 5(a), the vertical and horizontal axes indicate the level of an input signal and a control voltage, respectively. It is controlled so that the control voltage may increase if the input signal level increase and decrease if the input signal level decreases.
When it is considered that the current source 11 of the variable gain amplifier circuit 10 shown in FIG. 4 comprises a plurality of MOS transistors, at least three MOS transistors constituting the current source 11, the gain control MOS transistors 12 or 13, and signal amplification MOS transistors 14, 15, 16 or 17, all of which are connected in series. In this case, a power supply voltage VDD is required, which is a voltage more than three times the needed operation voltage for a MOS transistor. Therefore, the variable gain amplifier circuit 10 shown in FIG. 4 is not capable of being operated by a low power supply voltage.
In order to solve such a problem, for example, a variable gain amplifier circuit 20 shown in FIG. 6 can be considered.
This variable gain amplifier circuit 20 can control gain by changing the current of a current mirror circuit. In FIG. 6, the same reference numerals are attached to the same components as those as in FIG. 4, and their descriptions are omitted.
The variable gain amplifier circuit 20 shown in FIG. 6 can be obtained by using a current mirror circuit comprising n-channel MOS transistors 21, 22, 23 and 24, p-channel MOS transistors 25 and 26 and n-channel MOS transistors 14′-17′, instead of the p-channel MOS transistors 12 and 13 for controlling gain and p-channel MOS transistors 14-17.
The operation of n-channel MOS transistors 14′-17′ is basically the same of the p-channel MOS transistors 14-17 shown in FIG. 4.
In the variable gain amplifier circuit 20, if a control voltage applied to the gate of a p-channel MOS transistor 25, for example, the drain current of the p-channel MOS transistors 25 and 26 increases and decreases, respectively. Respective currents proportional to the drain currents of the p-channel MOS transistors 25 and 26 flow through the drains of the n-channel MOS transistors 21 and 22, and the drains of the n-channel MOS transistors 23 and 24, respectively.
Therefore, by controlling the drain current of the p-channel MOS transistor 25, the voltage difference between the drain voltages of the n-channel MOS transistors 14′ and 16′, and 15′ and 17′ can be changed, thereby controlling the gain of the variable gain amplifier circuit 20.
Patent reference 1: Japanese Patent Application Publication No. H05-29856 (FIG. 1)
Patent reference 2: Japanese Patent Application Publication No. H07-122950 (FIG. 1)